1. Field of the Invention
This invention is related to marine geophysical exploration and in particular to a vibratory source useful in marine geophysical exploration.
2. Background Art
Seismic sources, including vibratory sources, are utilized in geophysical exploration on land and in water covered areas of the earth. Signals generated by these sources travel downwardly into the earth and are reflected from reflecting interfaces in the subsurface and are detected by signal detectors, typically hydrophones or geophones, on or near the earth's surface.
Most of the acoustic sources employed today in marine operations are of the impulsive type, in which efforts are made to generate as much energy as possible during as short a time span as possible. The frequency content of such sources is controllable only to a small degree, and different sources are selected for the generation of different frequency ranges for different surveying needs. Vibratory acoustic sources, including hydraulically powered sources and sources employing piezoelectric or magnetostrictive material, have been used in marine operations. However, such sources have found only limited use. Although such sources can generate signals over various frequency bands, commonly referred to as “frequency sweeps”, the limited power that such sources known to the prior art have been able to generate have limited their use in marine operations.
It is well known that as sound waves travel through water and through subsurface geological structures, higher frequency sound waves are attenuated more rapidly than lower frequency sound waves, and consequently, lower frequency sound waves can be transmitted over longer distances through water and geological structures than higher frequency sound waves. There has for a long time been a need in the seismic sector of the oil and gas industry for powerful low frequency marine sound sources.
Accordingly, there is a continuing need for improved acoustic sources for use in marine operations.